1. Field of the Invention
The present invention relates to a structure of a brushless motor and, more particularly, to a brushless motor in which arc-shaped magnetic pole portions are arranged around a magnet rotor to thereby form a rectangular or arc shape as an overall shape.
2. Related Background Art
The same assignee as the present invention has already applied the above-mentioned arc-shaped brushless motor in U.S. patent application Ser. No. 027173, now U.S. Pat. No. 4,806,813.
In addition, as conventional brushless motors in which magnetic pole portions are arranged around a magnet rotor to thereby form an arc shape as an overall shape, there have been proposed brushless motors as disclosed in: Japanese Utility Model Application Nos. 59-26806, 59-26804, 56-107957, and 59-129607 (Japanese Utility Model Laid-open Nos. 60-140934, 60-141682, 58-15485, and 61-44637), Japanese Patent Application No. 59-39082 (JP-A-60-184234), and the like.
The foregoing conventional motor is used to control a sector of a shutter The motor comprises: a rotor which is rotatably supported and consisting of a permanent magnet which is magnetized by four poles in the radial direction; two stators each having a pair of U-shaped leg portions and in each of which a magnetic pole portion which faces the outer periphery of the rotor is formed in each end portion of the leg portions; and a coil to excite the stators. Further, the magnetic pole portions of the stators are formed so as to have a difference of 90.degree. between their phases to the rotational center of the rotor. In a state in which the two stators do not overlap with each other on a plane, one of the magnetic pole portions of each of the stators is mutually closely arranged There is a difference of 45.degree. between the phases of the closely arranged magnetic pole portions to the center of the rotor. The two stators are positioned so as to have a --character shape.
According to the motor with the structure such that the magnetic pole portions are formed at the edges of the U-shaped leg portions around the magnet rotor in the foregoing conventional techniques, in order to obtain the smooth rotation of the rotor, gaps (air gaps) between the rotor and the magnetic pole portions need to be uniform over the whole periphery of the rotor. Further, to raise the rotational torque, for the magnetic pole portions, in order to form opposite surfaces along the outer periphery of the rotor and to equalize the error gaps between the magnetic pole portions which construct each pair, a fixing method of the assembling positions of the rotor and leg portions also needs to be devised.